1. Field of the Invention
The present invention relates to a communication system using a telephone line, and more particularly, to an adaptive channel equalizing system and carrier recovery circuit in a home phoneline networking alliance (HomePNA) receiver.
2. Description of the Related Art
The HomePNA is an association of companies committed to promulgating a single, unified phoneline home networking standard. The HomePNA standard (hereinafter HomePNA) is a home networking solution using a single telephone line.
FIG. 1 is a schematic block diagram of a conventional equalizer and carrier recovery circuit system. Referring to FIG. 1, a receiver receiving a signal transmitted through a channel includes an equalizer 110, a decision device 120, a carrier recovery circuit 130, an error estimation device 140, and a mixer 150.
The equalizer 110 recovers a transmission signal distorted due to inter-symbol interference (ISI) and is realized as an adaptive equalizer. ISI occurs due to a band-limited channel characteristic of the telephone line.
The decision device 120 is a circuit that parses a signal from a transmission signal having one or more signals with various magnitudes and/or phases. The decision device 120 use a hard-decision method.
The carrier recovery circuit 130 compensates for a frequency error due to a disparity in oscillation frequency between a transmitter and a receiver or a phase error due to other causes. The carrier recovery circuit 130 receives an input signal from the mixer 150 and a signal output by the decision device 120 and estimates a carrier error from the two signals. The mixer 150 multiplies the carrier error output from the carrier recovery circuit 130 by an output signal of the equalizer 110 to counterbalance the carrier error. Consequently, any distortion due to the carrier error has been compensated for before the mixer 150 outputs a signal to the decision device 120.
The error estimation device 140 outputs an error signal to the equalizer 110 using an error between the reception signal before decision and the reception signal after decision and the carrier error estimated by the carrier recovery circuit 130. The equalizer 110 equalizes a reception signal r(t) using the error signal output from the error estimation device 140.
In the equalizer and carrier recovery circuit system shown in FIG. 1, the reception signal r(t) is channel-equalized by the equalizer 110 before the carrier error is compensated for by the carrier recovery circuit 130. The reception signal r(t) can be channel-equalized after being carrier error-compensated.
A conventional equalizer and carrier recovery circuit repeatedly performs channel equalization and carrier recovery on a reception signal according to a particular modulation mode applied to the reception signal.
In a HomePNA system according to a phoneline networking modem specification, different modulation modes exist within a single frame.
FIG. 2 is a diagram showing a frame structure in HomePNA. Referring to FIG. 2, a frame is divided into a header, a payload, and a trailer. The header is a start region of the frame comprising information about the frame and includes a preamble. The payload is a user data region, and the trailer is an end region of the frame. Only frequency diverse quadrature amplitude modulation (FD-QAM) is used for the header and the trailer, and either FD-QAM or quadrature amplitude modulation (QAM) can be used for the payload. In FD-QAM, a zero symbol is inserted between symbols.
FIG. 3 is a diagram showing symbols in FD-QAM and QAM. Since a zero symbol is inserted between data symbols, a baud rate in the FD-QAM is one half of that in the QAM. For example, while the QAM has a 4 M baud rate, the FD-QAM has a 2 M baud rate.
FIG. 3(a) shows a case in which FD-QAM symbols having a 2 M baud rate are received and then QAM symbols having a 4 M baud rate are received, and FIG. 3(b) shows a case in which QAM symbols having a 4 M baud rate are received and then FD-QAM symbols having a 2 M baud rate are received.
When different modulation modes exist within a single frame, as shown in FIGS. 2 and 3, a conventional equalizer circuit equalizes a modulated signal using a coefficient estimated by an equalizer. When a different modulated signal is received, the modulated signal cannot be appropriately equalized.
A conventional equalizer circuit of a HomePNA system starts channel equalization using a preamble. Since the preamble uses only FD-QAM, a data region (i.e., a payload region) using FD-QAM can be satisfactorily channel-equalized using a coefficient obtained by an equalizer while the preamble is received. However, when channel equalization is performed on a data region using QAM continuously using a coefficient obtained by an equalizer while the preamble is received, convergence cannot be secured and a reception signal cannot be demodulated.
In addition, a conventional carrier recovery circuit estimates a carrier error using all of the received symbols regardless of a modulation mode for a received signal. When all of the received symbols are used in estimating a carrier error, it is difficult to estimate the carrier error in a section in which an FD-QAM signal is received due to zero symbols.
When a residual carrier error remains because a carrier recovery circuit does not adequately estimate a carrier error, channel equalization may not converge. Conversely, when an equalizer cannot compensate for interference between residual adjacent symbols, the carrier recovery circuit is badly affected and thus cannot adequately estimate a carrier error.
Accordingly, an equalizer and carrier recovery circuit for reliably equalizing received symbol signals and reliably estimating and compensating for a carrier error, even if different modulation modes exist within a single frame, are desired.